The invention is directed to an assembly process for producing LED rows, particularly for the character generator of a non-mechanical printer.
What is usually meant by LED rows is the arrangement of LEDs, i.e. of light-emitting diodes in row form. Originally, discrete individual components were assembled to form rows. A coarse grid of, for example, 1/10, 1/20 inches of completely encapsulated LEDs could only be realized therewith. Later, individually unencapsulated LED chips were assembled to form rows; significantly finer grids could be realized therewith. At present, integration on the semiconductor wafer makes it possible to arrange LEDs with an extremely high dot density, for example, 300 or 600 dpi (dots per inch).
Due to limitations in the semiconductor wafer size and for reasons of yield, it is only possible to manufacture smaller strip sections of LED rows integrated in such fashion. In order to manufacture long LED rows, for example, for printing purposes, a plurality of such sections must be assembled fitting very exactly to one another.
LEDs are usually employed for display purposes. DE-A 35 34 338, for example, also already discloses, however, to employ LEDs for printing purposes in a character generator for a non-mechanical printer. The character generator disclosed therein contains a plurality of light sources fashioned, for example, as light-emitting diodes in an illumination row. A latent, electrostatic image is produced on a transfer-printing drum upon employment of the light sources. In the known character generator, all of the components that form the illumination row such as the light-emitting diodes, the drive circuits and leads, are firmly mounted on a shared carrier, whereby the fastening of the light-emitting diodes and drive circuits is undertaken by gluing. The carrier thereby has a length that is at least as great as the width of the entire illumination row.
In character generators, the employment of LEDs as imaging elements requires that the strictest tolerances be observed in all three directions given the high dot density of up to 600 dpi, i.e. with respect to division spacing, trueness to line and surface planarity. On the other hand, an optimum, thermal coupling to a carrier composed of metal is required for eliminating the high dissipated heat from the LEDs. These demands, however, cannot be met without further ado in the manufacturing methods up to now. On the basis of a preceding adjustment, the gluing of LED chips onto a carrier in fact enables the required observation of strictest tolerances in all three directions; the glue, however, prevents the optimum thermal coupling required for eliminating the dissipated heat. On the other hand, a soldering of the LED chips to a metal carrier enables an optimum thermal coupling; when soldering a LED chip, however, the softening or re-melting of the soldered connection of the neighboring LED chip necessarily leads to the de-adjustment thereof.